The changes of composition of the processed traditional Chinese medicine will affect the curative effect of drug, such as the four kinds of processed rhubarb. The characteristics data of each rhubarb was measured with terahertz spectroscopy system and analyzed with chemometrics, and the spectral data was classified according to the category of rhubarb. The substance components of anthraquinone and tannins make changes in processed rhubarb by thin layer chromatography (TLC). The correlation among the terahertz spectroscopy of processed rhubarb was in accordance with the variations of content. This means that terahertz spectroscopy is sensitive to the substance components of processed Chinese traditional medicine. It can also pave the way for the study of the structural changes of traditional Chinese medicine.